Secret signaling



June 16,. 1925. 1,542,565

R. c. MATHEs sEoRET SIGNALING Filed June 30. 1923 "Ioni" acting as somuch noise in the receiver.

Patented June 16, 1925.

UNITED STATES PATENT OFFICE.

ROBERT C. MTHES, OF NEW YORK, N. Y., ASSIG-NOR T( WESTERN ELECTRIC COM-PANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

SECRET SIGNALING.

Application filed .Tune 30, 1923. Serial No. 648,647.

To all whom z'zf may concern:

Be it known that I, ROBERT C. MATHES, a citizen of the United States ofAmerica, residing at New York city, in the county of Bronx and State ofNew York, have invented certain new and useful Improvements in SecretSignaling, of which the following is a full, clear, concise, and exactdescription.

The present invention relates to wave transmission with secrecy for anypurpose,j

such as for secret signaling.

The invention has particular application in transmitting and receivingsignals, or the like, comprised of a considerable band of frequencies,such as speech.

An object of the invention is to obtain a high degree of secrecy withcomparatively simple terminal apparatus and by such transformations ofthe signal or other waves as will preserve a high degree of quality inthe reproduction of the signals at the receiver.

A further object of the invention, when it is applied to radio orcarrier transmission, is to economize in the energy and in the breadthofthe frequency band transmitted.

Briefiy and specifically stated, according to the invention, the speechor other wave band is divided as to frequency into a plurality ofsub-bands, some of which may be transmitted without modification andothers of which first have their frequency order inverted and are thentransmitted along with the unchanged sub-bands, or, if desired, each ofthe sub-bands may be inverted beforf" transmission. Each of thesub-bands is preferably so chosen as vto be by itself in capable ofreproducing understandable speech. 'The possibility of understanding anyof the message contained in one of the sub-bands is still furtherlessened by the presence of a neighboring sub-band of invertedfrequencies, these inverted frequencies The message is received andunderstood at an authorized station by reinverting each in` vertedsub-band and properly combining it with the other sub-bands.

The various objects and features of the invention will be more fullyapparent from the following detailed description in which .referencewill be made to the accompanying drawing, showing inF ig. 1, a form oftransmitting' station employing the invention and For example, filter F,may

may transmit frequencies from 1500 to 2200 cycles, it being assumed thatthe essential speech frequencies from 100 to 2200 cycles are to betransmitted. This result may be accomplished by constructing each ofthese filters in the form of the wellknown band filter composed of anumber of sections as disclosed in the Campbell Patent No. 1,227,113,May 22, 1917. If desired, a different number of filters than the threeillustrated may be used to sub-divide the speech frequencies, in whichcase the band Width of the -respective filters will be suitably chosento correspond with the number of filters used.

In the form of the embodiment illustratedV in Fig. 1, the sub-band fromeach of the band filters is applied to an inverting modulator, thesebeing shown at M M2 and M3 respectively. For example, the sub-band fromthe filter F, is applied through the transformer 12 to the common branchon the input side of the modulator M, comprising :the discharge tubes 13and 14. A continuous wave of a frequency equal to the sum of the filterband edges, in this case 100 plus 800. cycles, equal to 900 cycles, issupplied from the source 15 to the individual input windings 16 and 17.The output windings 18 of the tubes 13 and 14 are connected in common tothe outgoing circuit 19 which includes a filter F, having the sametransmission frehquency band as the filter F,. The source 15 may be avacuum tube oscillator of the type shown in the Hartley Patent No.1,356,763 October 26, 1920 and the modulator M, is o the general typedisclosed in Carsons Patent No. 1,343,306, June 15, 1920.

The purpose of the modulator M,L and its associated circuits is toinvert the frequency order of the applied waves from the filter F, sothat in the outgoing circuit 19 only Wave components which have. afrequency order the inverse of that of the applied waves are present.The action of this modulator as is well known, is to produce an upperside band extending from 1000 to 1700 cycles, and a lower side bandextending from 100 cycles to 800 cycles, in which the 100 cyclecomponent is produced as the difference frequency between the 900 cyclecon tinuous wave and the corresponding 800 cycle component. Similarly,each component of the lower side band represents the difference infrequency between the 900A cycle continuous wave and a correspondingcomponent of the applied wave. As has been stated, the filter F1selectively transmits only the range 100 to 800 and, there* fore,transmits only the inverted frequencies.

In a similar manner,the wave transmitted throughthe filter F2 modulatesthe continu- 'ous wave from the source 20 of a fre uency of 2300 cycles,andthe filter F2 whic may be a duplicate of filter F2, selectivelytransmits the lower side band, that is, a wave having a frequency orderthe inverse of that applied to the modulator. The source 22 has afrequency of 3700 cycles and consequently when the Wave transmittedthrough lter F5 .is applied to the modulator M3, a lower side bandwithin the same frequency limits as the applied sub-band is producedhaving a frequency order the inverse of the applied sub- `band, and thisis selectively transmitted by the citer F3'.

Due to the fact that the sub-bands from .applied in common through thetransformer 23 to the input side of the transmitting modulator TM whichhas its output side associated with the transmitting antenna TA. Thecommon or neutral branch o f the input circuit is supplied with a radiofrequency wave from the source 24 so that this radio frequency wave ismodulated by a band of frequencies having a width'equal to the sum ofthe applied sub-bands. This source may be of the type disclosed in theHartley Patent No. 1,356,763 referred to above.

Due to the use of the balanced modulator TM, the unmodulated carrier orradiov wave component is balanced out in the output circuit of themodulator and is, therefore, prevented from being transmitted. Thetransmitting antenna TA. is preferably sharply tuned by means of thecondenser 25 so as to discriminate in favor of'one of the two side bandsof the modulated radio wave, andto suppress the other side band. Wherethe carrier or radio frequency is relatively low as, for example, 10,000to 20,000 cycles, this method of suppressing one side band is effective.If very high carrier frequencies are to be used, it is preferable, inorder to secure single side band transmission, to employ a modulatingsystem of the type shown in the Osborne Patent No. 1,361,488, December7, 1920, since at very high frequencies it is impracticable to providesufficiently sharp tuning in an antenna to discriminate materiallybetween speech side-bands.

It is afe'ature of the invention that it does not depend forsecrecy uponthe transmission of all of the components of the modulated Wave norcomponents in both side bands, nor transmission of the unmodulatedcarrier component, but that only one side band need betransmitted, andtherefore the energy contained in one of the side bands and in theunmodulated carrier component need not be transmitted. It has been foundthat the energy of one side band and the' unmodulated carrier componentcomprises about {ive-sixth of the total energy of the modulated wave andapplicant is enabled therefore to effect a considerable economy in theenergy transmitted by having to transmit only a single Side 4hand orsubstantially one-sixth of the energy of the entire modulated wave. Thisfeature makes the invention particularly adapted for long distancecommunication such as trans-atlantic radio telephony. However, theinvention is not limited to such use since the economy in the energy andfrequency range employed by the invention makes it advantageous forother types of transmission such as wire transmission, either by thecarrier current method or by transmission at the signal frequency level.

It is not necessary to invert -the frequency order of eachA of thesub-bands. For example, the frequency order may be inverted in Ithe caseof two of the sub-bands rovid-ed for in Fig. 1, while the other subliandmay be applied to the modulator TM without first being inverted. To dothis, it is only necessary to omit the modulating circuit M1 or M2 orM3, as the case may be, and to connect the corresponding dividing ltersuch as F1, F2 or F8 directly across to the circuit 19.

If the Wave transmitted from the station in Fig. 1, is received by asimple detector,

itis obvious that no components capable of reproducing intelligiblespeech will be obtained, since 'each ofthe sub-bands is, by itself,incapable 'of being understood, and the presence of the-invertedsub-band or sub-bands produces noise components which will interferewith the reception of such lil() fragmentary speech components as may bey present'incase' one or more' of the m1li-bands vthrough the filtersRF1 RF2 detector D ivith a continuous Wave of the radio or carrierfrequency supplied from the source 26. As a rusult of the Well knownaction of the detector D, the lou7 frequency output Waves containcomponents corresponding to those which were impressed on the modulatorTM at'the distant station. These components are separated andselectively transmitted through the respective filters RF1, RF2 and RF3to the inverting or demodulatingcircuits DMl, DM2 and DM3, respectively.These filters may be duplicates of the filters F1, F2 and F3,respectively, and the demodulators DMI, etc., may be duplicates of themodulator M1 of Fig. 1. Also, the three Wave sources 27 28 and 29 may beduplicates of the respective Wave sources 15, 20 and 22 of Fig. 1 andare adjusted to have the same respective fre quencies.

As explained in connection with Fig. 1, therefore, the action of thedemodulating circuits is to invert the frequency orders of the impressedsub-bands which, it will be recalled, have frequency orders the inverselof the normal frequency order of the subbands as derived from speech.The reinverted sub-bands are selectively transmitted and RF3 to thereceivers R. The sub-bands applied in common to the receiver R, it Willbe seen, are substantially the same as the three sub-ban ds divided outof normal speech by the' filters F1, F2 and F3' of Fig. l and thesereceived and reinverted sub-bands produce understandable speech in thereceiver R.

No amplifiers have been shown, but, it Will be obvious to supply thenecessary amplification at suitable points in the system.

That i's claimed is:

1. The method of transmission of Waves having a band of frequencyvcomponents occurring in a characteristic order, which comprises dividingsaid band into sub-bands, inverting the frequency order of thecomponents of certain of the sub-bands while keeping the respectivesub-bands Within substantially the same frequency limits as they occupybefore inversion, and transmitting all of said sub-bands simultaneously.

2. The method of secret transmission of speech comprising sub-dividing.the speech waves into sub-bands of frequency components, the componentscomprised in any one sub-band being incapable of reproducingunderstandable speech, inverting the frequency order of certain of thesub-bands While keeping the sub-bands Within substantially the samefrequency limits, and transmitting all of the sub-bands simultaneously.

3. The method of receiving the Waves secretly transmitted by the methoddefined n in claim 1, comprising receiving all of the transmittedsub-bands simultaneously, reinverting the frequency order of each of thesub-bands of inverted frequency to restore the characteristic frequencyorder of each sub-band, and applying the restored subbands together to areceiver.

4. The method of receiving understandable speech from the Waves secretlytransmitted by the method defined in claim 2 which comprises receivingall of the transmitted sub-bands simultaneously, separating theindividua-l sub-bands by filtering, rein-V verting the frequency orderof each of the inverted frequency sub-bands to restore the normal orderof frequencies Within each subband, and recombining the restoredsubbands and applying them simultaneously to a receiver.

5. The method of secret carrier Wave transmission comprising modulatinga carrier Wave by the sub-bands resulting from the method defined inclaim 1 and transmitting the modulated Wave to-a dist-ance.

6. The method of secret transmission of speech comprising subdividingthe speech waves into sub-bands of frequency components, the componentscomprised in any one sub-band being. incapable iof reproducingunderstandable speech, inverting the frequency order of certain of thesub-bands While keeping the sub-bands Within substantially the samefrequency limits, modulating a carrier wave by the sub-bands soproduced, suppressing transmission of the unmodulated carrier componentand one of the two side-bands resulting from the modu lation, andtransmit-ting the remaining sideband to a distance.

.7. The method of reproducing understandable speech Waves from the sideband transmitted by the method'defined in claim 6 which comprisescombining the side band with a Wave of the carricr'frequency in adetecting circuit, separating the individu-al sub-bands Within speechrange obtained from the detection, reinverting the frequency order ofeach inverted frequency subband, and combining the resulting subbands ina receiver.

8. The method' of securing secrecy of transmiion Within the frequencylimits of a single side-band of a modulated Wave, comprising modulatinga carrier Wave by suppressing the unmodulated carrier component and oneside-band, and transmitting the remaining side-band.

9. The method of secret radio telephony comprising transposing essentialspeech requency components to produce a band of unintelligiblecomponents occupying substantially the same frequency limits as theoriginal speech frequency components, modulating a radio wave by theband of frequency components so transposed, suppressing 10 transmissionof one of the two resulting side-bands of the modulated Wave and theunmodulated carrier Jfrequency component, and transmitting the remainingside-band to a distance. 15 In Witness whereof, I hereunto subscribe myname this 26th day of June A. D., 1928.

ROBERT C. MATHES.

